Packet ring networks are typically significantly easier to operate and administer than complex mesh or irregular networks, and a ring network may also allow for failure of a link between nodes of the network, if the network is bi-directional. The leading bi-directional protocol for high speed packet rings is the Resilient Packet Ring (RPR) protocol, defined by IEEE Standard 802.17.
If services provided by the ring network do not require guaranteed bandwidth, all nodes (termed stations in RPR) operating in the ring may use a “Best Effort” approach to transfer packets. To meet guarantee needs, however, RPR allocates guaranteed bandwidth for class A traffic and some class B traffic. (RPR defines three classes of traffic: class A, class B, and class C. Class A is a low latency, low jitter class.) Class A0 is a subdivision of class A. The bandwidth of a class A0 traffic reservation may only be used by the station holding the reservation, and any such reserved bandwidth that is unused is wasted.
At initiation of an RPR network a station on the RPR network broadcasts reservation requests for its class A0 traffic using topology messages (which are also used for stations to notify each other of their existence and position in the ring). The reservations are typically determined by Service Level Agreements (SLAs) between users and an operator of the ring. A Connection Admission Controller (CAC) in the network allocates bandwidth according to the received requests, and all stations on the RPR are informed of the allocation. For any new service, the CAC must know how much bandwidth has been consumed, and how much is required by the new service, to verify that the new service can be provisioned according to the new service's SLA. In addition, and regardless of the class of traffic, it may be necessary to reserve some bandwidth to avoid traffic starvation.